Modulator-induced interference in functional cross talk between the substrate and the ATP sites of human P-glycoprotein

On interactions of P-glycoprotein with various anti-tumor drugs by binding free energy calculations

P-glycoprotein (P-gp, MDR1), one of ATP-binding cassette transporters, may confer tumor cells cross-resistance to chemotherapeutics. A large amount of P-gp inhibitors were designed to inhibit the multidrug resistance (MDR) feature of P-gp. However, no sufficient researches were reported to explore the correlation between binding capacity and drug property by experiment. Without particular drug property found to inhibit the MDR feature of P-gp, the orientation of drug design is indefinite. In this work, 10 representative cancer drugs with various properties are used to bind with P-gp by molecular dynamics simulation. Binding free energy between P-gp and 10 drugs ranges -139 to -253 kJ/mol. It reveals that the promiscuity nature of P-gp is in light of the similar binding free energy in separate P-gp-ligand binding systems. The binding effect of P-gp and drugs correlates well with the size of drugs and has no apparent correlation with the polarity of each drug. The key reason is that van der Waal's interaction occupies most of the total binding free energy, and it is led by the number of atoms in the drugs. Two transmembrane segments (TM6 and TM12) and three types of amino acids (PHE, MET, and GLN) are vital in binding drugs with van der Waal's energy, which evident the influence between binding stability and size of drugs. This work provides the cause and theoretical basis for the promiscuity nature of P-gp.Communicated by Ramaswamy H. Sarma.

Candida albicans Antifungal Resistance and Tolerance in Bloodstream Infections: The Triad Yeast-Host-Antifungal

Candida albicans represents the most frequent isolated yeast from bloodstream infections. Despite the remarkable progress in diagnostic and therapeutic approaches, these infections continue to be a critical challenge in intensive care units worldwide. The economic cost of bloodstream fungal infections and its associated mortality, especially in debilitated patients, remains unacceptably high. Candida albicans is a highly adaptable microorganism, being able to develop resistance following prolonged exposure to antifungals. Formation of biofilms, which diminish the accessibility of the antifungal, selection of spontaneous mutations that increase expression or decreased susceptibility of the target, altered chromosome abnormalities, overexpression of multidrug efflux pumps and the ability to escape host immune defenses are some of the factors that can contribute to antifungal tolerance and resistance. The knowledge of the antifungal resistance mechanisms can allow the design of alternative therapeutically options in order to modulate or revert the resistance. We have focused this review on the main factors that are involved in antifungal resistance and tolerance in patients with C. albicans bloodstream infections.

Two inhibitors of yeast plasma membrane ATPase 1 (ScPma1p): toward the development of novel antifungal therapies

Given that many antifungal medications are susceptible to evolved resistance, there is a need for novel drugs with unique mechanisms of action. Inhibiting the essential proton pump Pma1p, a P-type ATPase, is a potentially effective therapeutic approach that is orthogonal to existing treatments. We identify NSC11668 and hitachimycin as structurally distinct antifungals that inhibit yeast ScPma1p. These compounds provide new opportunities for drug discovery aimed at this important target.

A reciprocating twin-channel model for ABC transporters

ABC transporters comprise a large, diverse, and ubiquitous superfamily of membrane active transporters. Their core architecture is a dimer of dimers, comprising two transmembrane (TM) domains that bind substrate, and two ATP-binding cassettes, which use the cell's energy currency to couple substrate translocation to ATP hydrolysis. Despite the availability of over a dozen resolved structures and a wealth of biochemical and biophysical data, this field is bedeviled by controversy and long-standing mechanistic questions remain unresolved. The prevailing paradigm for the ABC transport mechanism is the Switch Model, in which the ATP-binding cassettes dimerize upon binding two ATP molecules, and thence dissociate upon sequential ATP hydrolysis. This cycle of nucleotide-binding domain (NBD) dimerization and dissociation is coupled to a switch between inward- or outward facing conformations of a single TM channel; this alternating access enables substrate binding on one face of the membrane and its release at the other. Notwithstanding widespread acceptance of the Switch Model, there is substantial evidence that the NBDs do not separate very much, if at all, and thus physical separation of the ATP cassettes observed in crystallographic structures may be an artefact. An alternative Constant Contact Model has been proposed, in which ATP hydrolysis occurs alternately at the two ATP-binding sites, with one of the sites remaining closed and containing occluded nucleotide at all times. In this model, the cassettes remain in contact and the active sites swing open in an alternately seesawing motion. Whilst the concept of NBD association/dissociation in the Switch Model is naturally compatible with a single alternating-access channel, the asymmetric functioning proposed by the Constant Contact model suggests an alternating or reciprocating function in the TMDs. Here, a new model for the function of ABC transporters is proposed in which the sequence of ATP binding, hydrolysis, and product release in each active site is directly coupled to the analogous sequence of substrate binding, translocation and release in one of two functionally separate substrate translocation pathways. Each translocation pathway functions 180° out of phase. A wide and diverse selection of data for both ABC importers and exporters is examined, and the ability of the Switch and Reciprocating Models to explain the data is compared and contrasted. This analysis shows that not only can the Reciprocating Model readily explain the data; it also suggests straightforward explanations for the function of a number of atypical ABC transporters. This study represents the most coherent and complete attempt at an all-encompassing scheme to explain how these important proteins work, one that is consistent with sound biochemical and biophysical evidence.

Evidence for modulatory sites at the lipid-protein interface of the human multidrug transporter P-glycoprotein

The human multidrug transporter P-glycoprotein (Pgp or ABCB1) sets up pharmacological barriers to many clinically important drugs, a therapeutic remedy for which has yet to be formulated. For the rational design of mechanism-based inhibitors (or modulators), it is necessary to map the potential sites for modulator interaction and understand their modes of communication with the other functional domains of Pgp. In this study, combining directed mutagenesis with homology modeling, we provide evidence of two modulator-specific sites at the lipid protein interface of Pgp. Targeting 21 variant positions in the COOH-terminal transmembrane (TM) regions, we find residues M948 (in TM11) and F983, M986, V988, and Q990 (all four in TM12) critically involved in substrate-site modulation by a thioxanthene-based allosteric modulator cis-(Z)-flupentixol. Interestingly, for ATP-site modulation by the same modulator, only two (M948 and Q990) of those four residues appear indispensable, together with two additional residues, T837 and I864 in TM9 and TM10, respectively, suggesting independent modes of communication linking the allosteric site with the substrate binding and ATPase domains. None of the seven residues identified prove to be critical for modulation of the substrate or ATP sites by Pgp modulators that are transported by the pump, such as cyclosporin A or verapamil, indicating their specificity for cis-(Z)-flupentixol. On the other hand, ATP-site modulation by verapamil proves to be highly sensitive to replacement at positions F716 (in TM7) and I765 (in TM8), and to a more moderate extent at I764 and L772 (both in TM8). Homology modeling based on the known crystal structures of the bacterial multidrug transporter SAV1866 and the mouse Pgp homologue maps the identified residues primarily at the lipid-protein interface of Pgp, in two spatially distinct modulator-specific clusters. The two modulatory sites demonstrate negative synergism in influencing ATP hydrolysis, consolidating their spatial distinctness. Because Pgp is known to recruit drug molecules directly from the lipid bilayer, identification of modulatory sites at the lipid-protein interface and at the same time outside the conventional central drug binding cavity is mechanistically revealing.

The monoamine oxidase A inhibitor clorgyline is a broad-spectrum inhibitor of fungal ABC and MFS transporter efflux pump activities which reverses the azole resistance of Candida albicans and Candida glabrata clinical isolates

Resistance to the commonly used azole antifungal fluconazole (FLC) can develop due to overexpression of ATP-binding cassette (ABC) and major facilitator superfamily (MFS) plasma membrane transporters. An approach to overcoming this resistance is to identify inhibitors of these efflux pumps. We have developed a pump assay suitable for high-throughput screening (HTS) that uses recombinant Saccharomyces cerevisiae strains hyperexpressing individual transporters from the opportunistic fungal pathogen Candida albicans. The recombinant strains possess greater resistance to azoles and other pump substrates than the parental host strain. A flow cytometry-based HTS, which measured increased intracellular retention of the fluorescent pump substrate rhodamine 6G (R6G) within yeast cells, was used to screen the Prestwick Chemical Library (PCL) of 1,200 marketed drugs. Nine compounds were identified as hits, and the monoamine oxidase A inhibitor (MAOI) clorgyline was identified as an inhibitor of two C. albicans ABC efflux pumps, CaCdr1p and CaCdr2p. Secondary in vitro assays confirmed inhibition of pump-mediated efflux by clorgyline. Clorgyline also reversed the FLC resistance of S. cerevisiae strains expressing other individual fungal ABC transporters (Candida glabrata Cdr1p or Candida krusei Abc1p) or the C. albicans MFS transporter Mdr1p. Recombinant strains were also chemosensitized by clorgyline to other azoles (itraconazole and miconazole). Importantly, clorgyline showed synergy with FLC against FLC-resistant C. albicans clinical isolates and a C. glabrata strain and inhibited R6G efflux from a FLC-resistant C. albicans clinical isolate. Clorgyline is a novel broad-spectrum inhibitor of two classes of fungal efflux pumps that acts synergistically with azoles against azole-resistant C. albicans and C. glabrata strains.

Efflux-mediated antifungal drug resistance

Fungi cause serious infections in the immunocompromised and debilitated, and the incidence of invasive mycoses has increased significantly over the last 3 decades. Slow diagnosis and the relatively few classes of antifungal drugs result in high attributable mortality for systemic fungal infections. Azole antifungals are commonly used for fungal infections, but azole resistance can be a problem for some patient groups. High-level, clinically significant azole resistance usually involves overexpression of plasma membrane efflux pumps belonging to the ATP-binding cassette (ABC) or the major facilitator superfamily class of transporters. The heterologous expression of efflux pumps in model systems, such Saccharomyces cerevisiae, has enabled the functional analysis of efflux pumps from a variety of fungi. Phylogenetic analysis of the ABC pleiotropic drug resistance family has provided a new view of the evolution of this important class of efflux pumps. There are several ways in which the clinical significance of efflux-mediated antifungal drug resistance can be mitigated. Alternative antifungal drugs, such as the echinocandins, that are not efflux pump substrates provide one option. Potential therapeutic approaches that could overcome azole resistance include targeting efflux pump transcriptional regulators and fungal stress response pathways, blockade of energy supply, and direct inhibition of efflux pumps.

The yeast Pdr5p multidrug transporter: How does it recognize so many substrates?

Multidrug transporters are of considerable importance because they present problems in the treatment of infectious disease and cancer. A central issue is the ability of efflux pumps to recognize an astounding array of structurally diverse compounds. The yeast Pdr5p efflux pump, which is a member of the ATP-binding cassette superfamily, has at least 3 substrate-binding sites, each of which appears to use different chemical properties to transport compounds. All Pdr5p substrates, however, have a size requirement that is independent of hydrophobicity.

Differential effects of the immunosuppressive agents cyclosporin A, tacrolimus and sirolimus on drug transport by multidrug resistance proteins

PURPOSE

We sought to determine the effects of the immunosuppressants, cyclosporin A (CsA), tacrolimus and sirolimus, on drug transport by the ATP-binding cassette proteins, P-glycoprotein (Pgp; ABCB1), multidrug resistance protein-1 (MRP-1; ABCC1) and breast cancer resistance protein (BCRP; ABCG2), and the major vault protein lung resistance protein (LRP).

METHODS

Cellular content of mitoxantrone, a Pgp, MRP-1 and BCRP substrate, was measured by flow cytometry in cells overexpressing these proteins following incubation with and without CsA, tacrolimus or sirolimus. Interaction of BCRP with these compounds was studied by photolabeling and ATPase assays. Nuclear-cytoplasmic distribution of doxorubicin was studied by confocal microscopy in cells overexpressing LRP.

RESULTS

CsA increased cellular drug uptake in cells overexpressing Pgp, MRP-1 or BCRP and nuclear drug uptake in cells overexpressing LRP at the clinically achievable concentration of 2.5 microM. Tacrolimus enhanced cellular drug uptake at 1 microM, but not at 0.08 microM, its clinically achievable concentration, and did not enhance nuclear drug uptake. Sirolimus enhanced cellular drug uptake in cells overexpressing Pgp, MRP-1 and BCRP with optimal effects at 2.5 microM, but was effective at its clinically achievable concentration of 0.25 microM if cells were pre-incubated for at least 30 min before drug exposure, and also enhanced nuclear drug uptake at 0.25 microM. BCRP modulation by all three immunosuppressive agents was associated with competitive binding to the drug transport sites.

CONCLUSIONS

CsA, tacrolimus and sirolimus modulate drug transport by Pgp, MRP-1 and BCRP and CsA and sirolimus modulate drug transport by LRP at concentrations that differ from immunosuppressive concentrations and maximum tolerated concentrations.

Gomisin A alters substrate interaction and reverses P-glycoprotein-mediated multidrug resistance in HepG2-DR cells

Through an extensive herbal drug screening program, we found that gomisin A, a dibenzocyclooctadiene compound isolated from Schisandra chinensis, reversed multidrug resistance (MDR) in Pgp-overexpressing HepG2-DR cells. Gomisin A was relatively non-toxic but without altering Pgp expression, it restored the cytotoxic actions of anticancer drugs such as vinblastine and doxorubicin that are Pgp substrates but may act by different mechanisms. Several lines of evidence suggest that gomisin A alters Pgp-substrate interaction but itself is neither a Pgp substrate nor competitive inhibitor. (1) First unlike Pgp substrates gomisin A inhibited the basal Pgp-associated ATPase (Pgp-ATPase) activity. (2) The cytotoxicity of gomisin A was not affected by Pgp competitive inhibitors such as verapamil. (3) Gomisin A acted as an uncompetitive inhibitor for Pgp-ATPase activity stimulated by the transport substrates verapamil and progesterone. (4) On the inhibition of rhodamine-123 efflux the effects of gomisin A and the competitive inhibitor verapamil were additive, so were the effects of gomisin A and the ATPase inhibitor vanadate. (5) Binding of transport substrates with Pgp would result in a Pgp conformational change favoring UIC-2 antibody reactivity but gomisin A impeded UIC-2 binding. (6) Photocrosslinking of Pgp with its transport substrate [125I]iodoarylazidoprazosin was inhibited by gomisin A in a concentration-dependent manner. Taken together our results suggest that gomisin A may bind to Pgp simultaneously with substrates and alters Pgp-substrate interaction.

Biochemical and pharmacological properties of an allosteric modulator site of the human P-glycoprotein (ABCB1)

The drug-transport function of the human P-glycoprotein (Pgp or ABCB1) is inhibited by a number of structurally unrelated compounds, known as modulators or reversing agents. Among them, the thioxanthene derivative flupentixol inhibits Pgp-mediated drug transport by an allosteric mechanism. Unlike most other Pgp modulators, the cis isomer of flupentixol [cis-(Z)-flupentixol] facilitates interaction of Pgp with its transport-substrate [125I]iodoarylazidoprazosin (or [125I]IAAP), yet inhibits transport. In this study, we show that the flupentixol site acts as a common site of interaction for the tricyclic ring-containing modulators thioxanthenes and phenothiazines. The allosteric stimulation of [125I]IAAP binding to Pgp occurs independent of the phosphorylation status of the transporter. Stimulation is retained in purified Pgp reconstituted into proteoliposomes, suggesting no involvement of any other cellular protein in the phenomenon. However, perturbation of the lipid environment of the reconstituted Pgp by nonionic detergent octylglucoside abolishes stimulation by cis-(Z)-flupentixol of [125I]IAAP binding. Extensive trypsin digestion of the [125I]IAAP-labeled Pgp generates a 5.5 kDa fragment with 80% of the stimulated level of labeling associated with it. Sensitivity to inhibition by transport-substrate vinblastine and competitive modulator cyclosporin A suggests that the elevated level of [125I]IAAP binding to the fragment represents a functionally relevant interaction with the substrate site of Pgp. In summary, we demonstrate that allosteric modulation by cis-(Z)-flupentixol is mediated through its interaction with Pgp at a site specific for tricyclic ring-containing Pgp modulators of thioxanthene and phenothiazine backbone, independent of other cellular components and the phosphorylation status of the protein.

Allosteric modulation of the human P-glycoprotein involves conformational changes mimicking catalytic transition intermediates

The drug transport function of human P-glycoprotein (Pgp, ABCB1) can be inhibited by a number of pharmacological agents collectively referred to as modulators or reversing agents. In this study, we demonstrate that certain thioxanthene-based Pgp modulators with an allosteric mode of action induce a distinct conformational change in the cytosolic domain of Pgp, which alters susceptibility to proteolytic digestion. Both cis and trans-isomers of the Pgp modulator flupentixol confer considerable protection of an 80 kDa Pgp fragment against trypsin digestion, that is recognized by a polyclonal antibody specific for the NH(2)-terminal half to Pgp. The protection by flupentixol is abolished in the Pgp F983A mutant that is impaired in modulation by flupentixols, indicating involvement of the allosteric site in generating the conformational change. A similar protection to an 80 kDa fragment is conferred by ATP, its nonhydrolyzable analog ATPgammaS, and by trapping of ADP-vanadate at the catalytic domain, but not by transport substrate vinblastine or by the competitive modulator cyclosporin A, suggesting different outcomes from modulator interaction at the allosteric site and at the substrate site. In summary, we demonstrate that allosteric interaction of flupentixols with Pgp generates conformational changes that mimic catalytic transition intermediates induced by nucleotide binding and hydrolysis, which may play a crucial role in allosteric inhibition of Pgp-mediated drug transport.